A conventional image sensor has a structure in which sensors that generate photocurrent signals depending on amounts of incident light are arranged in a two-dimensional array. Each sensor generates a current change as a signal component, and a signal reading circuit accumulates a charge of the current signal from each of the sensors, converts the current signal into a voltage, and successively outputs a read signal by scanning the voltage value.
FIG. 1 is a circuit diagram illustrating a part of an example of the conventional image sensor. FIG. 1 illustrates the signal reading circuit of the image sensor. In FIG. 1, each sensor part 1 has the same structure including a sensor 2, an input gate 3, a capacitance reset switch 4, and an integrating capacitor 5. An input part 11 includes the sensor part 1, an output amplifier 12, and a row selection switch 13. An external gate control signal PIG is input to a gate of the input gate 3, and an external reset signal Rst is input to a gate of the capacitance reset switch 4. A gate of the row selection switch 13 is connected to a vertical scan shift register 21. A power supply voltage VD is applied to the output amplifier 12. An external bias voltage VB is applied to a gate of a switch 15, and the output amplifier 12 and the row selection switch 13 are connected to an output line via a column selection switch 16. A gate of the column selection switch 16 is connected to a horizontal shift register 22. In FIG. 1, GND denotes a ground voltage, and an output voltage V(out) is output from an output terminal 25. In addition, the signal reading circuit may have a structure including the sensor 2 or, may have a structure excluding the sensor 2.
An amount of current of a photocurrent generated from the sensor 2 is converted into an amount of charge accumulated in the integrating capacitor 5 by turning ON the input gate 3 for a predetermined time by the external gate control signal PIG. A resulting voltage that is introduced across both terminals of the integrating capacitor 5 is regarded as a read signal. The output voltages (or read signals) that are generated from each of the sensor parts 1 that are arranged in the two-dimensional array are scanned time-sequentially by selecting the row selection switch 12 and the column selection switch 16 in an order using the vertical scan shift register 21 and the horizontal scan shift register 22. The scanned output voltages (or read signals) are successively output via the output terminal 25 to the outside as the output voltage V(out).
In the signal reading circuit illustrated in FIG. 1, the photocurrent from the sensor 2 is integrated as an amount of charge, and thus, the signal is proportional to C1 while the noise is proportional to C1/2, where C denotes the amount of charge. For this reason, the Signal to Noise Ratio (SNR) of the output of each sensor part 1 becomes larger as the amount of charge that can be accumulated in the integrating capacitor 5 becomes larger. Hence, the SNR improves as the capacitance of the integrating capacitor 5 within the sensor part 1 increases. However, in the case where the sensor parts 1 are arranged in the two-dimensional array, the area that may be occupied by the circuit part within the sensor part 1 becomes limited depending on the design conditions of the image sensor, and thus, there is a limit to improving the SNR of the output obtained from the signal reading circuit. In other words, in order to improve the SNR of the output obtained from the signal reading circuit, it is desirable to make the integrating capacitor 5 large, but at the same time, it is desirable to reduce the area occupied by the circuit part within the sensor part 1. Therefore, improving the SNR and reducing the area occupied by the circuit part within the sensor part 1 are in a tradeoff relationship.